Formulation suitable for ink receptive coatings

ABSTRACT

Formulations comprising novel porous metal oxide particles and binder are particularly suitable for ink receptive coatings, e.g., for ink jet papers and films. The metal oxide particles used in this application have a porous structure that differs significantly from the nonporous silica colloids. The particles have a median particle size in the range of about 0.05 to about 3 microns and porosity such that when an aqueous dispersion of the particles is dried at least 0.5 cc/g of pore volume is from pores having a pore size of 600 Å or less. The particles also have a viscosity derived pore volume of at least 0.5 cc/g. 
     Formulations comprising particles having a zeta potential of +20 mV are also disclosed.

This is a division of application Ser. No. 09/112,540, filed Jul. 9,1998.

BACKGROUND OF THE INVENTION

This invention relates to formulations comprising inorganic oxideparticles. In particular, this invention relates to ink receptivecoating formulations for paper comprising a novel porous, fine inorganicoxide which provides excellent ink absorption properties, and ifdesired, glossy finishes.

Ink receptive coatings typically contain various proportions ofinorganic pigments and binder(s). The proportions of these componentsaffect the properties of these coatings, e.g., ink absorptionproperties. One means of characterizing the proportion of inorganicpigment relative to the proportion of binder is by the pigment volumeconcentration, or PVC. The definition of pigment volume concentration(PVC) is: 100*V_(p)/(V_(p)+V_(b)), where V_(p) is the volume of thepigment and V_(b) is the volume of the binder.

When a coating is formulated at low PVC, the binder constitutes thecontinuous phase of the coating within which pigment particles aredispersed. When a coating is formulated at high PVC, the binder phase isno longer a continuous phase, that is, there is not enough binder tofill the voids between packed and semi-rigid or rigid pigment particles.The proportion at which the binder is no longer considered thecontinuous phase is referred to in the art as the critical pigmentvolume concentration (CPVC). At proportions above CPVC, a network ofinterparticle pores form between the close packed particles, and theseinterparticle pores become reservoirs for ink that is subsequentlyapplied to the dried coating.

It also is well known in the art that glossy and permeability propertiesof coatings comprising inorganic pigment and binder depend on the PVC.See Outlines of Paint Technology 3rd ed., W. M. Morgans, Halsted Press(a division of John Wiley & Sons, Inc.) New York, N.Y., 1990, p.7. Whena coating is formulated below the CPVC, a matte effect is created byimparting surface roughness and can be generally attained when thepigment particle size is large when compared to the coating thickness.Relatively glossy coatings can be achieved when the pigment particlesare small when compared to the coating thickness. Thus, matte finishesand coating gloss can be controlled by judicious choice of pigmentparticle size in relation to the coating film thickness. However, unlessthe binder is hygroscopic, such coatings will be relatively impermeableto water. For an ink-jet printing application with aqueous inks, such acoating would suffer from the shortcoming of relatively long inkdry-time. A key attribute of ink-receptive coatings is the ability toabsorb the ink fluid rapidly so that the image becomes fixed to themedia as quickly as possible. This minimizes smudging.

One known formulation comprising hygroscopic binders is an ink-receptivecoating formulated at low PVC using colloidal silica as the pigment inconjunction with hygroscopic binders such as PVOH. These formulationsgenerally result in relatively glossy coatings and the hygroscopicbinders absorb moisture via partial solubility of the ink-fluid. Thecolloidal silica in this instance serves to modify the coatingproperties to improve the image characteristics once the coatings isprinted. However, colloidal silica is non-porous and coatings preparedfrom colloidal silicas are relatively dense. Such a coating thereforelacks capacity to absorb large quantities of liquid ink. Furthermore,ink-drytimes are relatively slow.

When coatings are formulated at high PVC (and specifically, above theCPVC), it is typical to observe very different gloss and moisturepermeability properties than when formulated below the CPVC. At highPVC, a network of interstitial void space, or pores between theparticles, is created by imperfect packing of the particles and the lackof a suitable amount of binder to fill the interstitial voids. Suchcoatings tend to display a high degree of moisture permeability becauseof liquid flow through the interstitial pores, which is desirable forthe ink-receptive coating application. However, the gloss of suchcoatings is usually relatively low because the surface exhibits a degreeof roughness that is related to the pigment particle size. It ispossible to obtain relatively glossy coatings in this system, butrelatively small pigment particles are necessary and typicallyrelatively non-porous particles are used. For example, colloidal silicadescribed in European Patent Application 803,374 can be used for glossyapplications. Colloids derived from fumed silica also have been used forthis purpose, but those materials are also non-porous.

Other pigments such as clays, aluminas, diatomaceous earth, precipitatedsilicas, etc., disclosed in U.S. Pat. Nos. 4,460,637 and 5,030,286, areused as well. While some of these pigments do have internal porosity,that porosity is subject to substantial reduction when the coatingcontaining the pigments is applied and dried.

Accordingly, there is a need to provide pigments for glossy inkreceptive coatings in which the inorganic particles contain internalporosity, regardless of the coating PVC. It also is desirable to havepigment porosity and hence coating porosity which is not as affected byexternal factors, such as shear, and is reliably present even after thepigment and coating is processed and dried.

SUMMARY OF THE INVENTION

The formulations of this invention comprise binder and porous inorganicoxide particles or pigments which have a median particle size in therange of 0.05 to about 3 microns. Unlike prior art colloidal particles,the particles of this invention have a porous structure such that atleast about 0.5 cc/g of the pore volume is from pores having a pore sizeof 600 Å or less. Porosity from pores less than 600 Å is referred toherein as internal porosity, i.e., porosity present in the particlesthemselves. Indeed, the internal porosity is reflected by a “viscosityderived pore volume”, defined later below, of at least about 0.5 cc/g.There also are embodiments comprising silica gel particles in which atleast about 0.7 cc/g and at least 0.9 cc/g of pore volume is from poreshaving sizes less than 600 Å. In these embodiments, at least 80% of thepore volume is from pores having pore sizes less than 300 Å.

The internal porosity of the particles in this invention is relativelystable and unlike prior art precipitated silicas is less susceptible tocomplete collapse under capillary pressures created when the waterevaporates from the dispersion during drying.

The formulations can be coated onto substrates and dried to form aporous layer which is particularly suitable as an ink receptive layer,e.g., ink jet paper. The dried layer resulting from the formulationgenerally has good ink absorption properties. Embodiments comprisingparticles having a median particle size in the range of 0.05 to 1 microncan be used to prepare relatively high gloss finishes, particularly forphotorealistic printing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of $\frac{\eta_{0}}{\eta}$

versus mass fraction solids for dispersions of particles used in severalembodiments of the invention and prior art colloidal silica, wherein ηis the viscosity of the dispersions illustrated and η₀ is the viscosityof water. Mass fraction solids include undissolved particles and doesnot include dissolved salts.

FIG. 2A shows a graph of $\frac{\eta_{0}}{\eta}$

versus mass fraction solids wherein η is the viscosity of dispersions ofparticles used in one embodiment of the invention wherein the particlescomprise a hydrous silica gel, and η₀ is the viscosity of water. (◯)represents data for viscosity and loadings before milling. (□)represents data for viscosity and loadings after being milled, and (Δ)represents data for viscosities and loadings of dispersions after beingmilled and centrifuged at 600 G's.

FIG. 2B is a graph of the same data for precipitated silica commerciallyavailable as Zeothix™177, where (◯) and(□) represents the same type ofdata indicated for FIG. 2A. (Δ) represents data for a dispersion whichhad been milled and centrifuged at 2,000 G.

FIG. 2C is a graph of the same data generated for FIG. 2A, but isgenerated for a precipitated silica commercially available as FK310 fromDegussa. (◯), (□) and (Δ) represent the same type of data indicated forFIG. 2A.

FIG. 3 is graph correlating viscosity derived pore volume (PVa) anddried pore volume measurements for particles used in the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) Preparation of Inorganic Oxide Particles

The inorganic oxide particles used in this invention can be preparedfrom conventional inorganic oxide materials. Suitable inorganic oxidesinclude precipitated inorganic oxides and inorganic oxide gels. Theseinorganic oxides are referred to herein as “parent inorganic oxides,”“parent particles” or “parent dispersions”. Amorphous precipitatedsilica and silica gels are particularly suitable parent inorganicoxides. The particles can also be prepared from mixed inorganic oxidesincluding SiO₂.Al₂O₃, MgO.SiO₂.Al₂O₃. The mixed inorganic oxides can beprepared by conventional blending or cogelling procedures.

Suitable inorganic oxide gels include, but are not limited to, gelscomprising SiO₂, Al₂O₃, AlPO₄, MgO, TiO₂, and ZrO₂,. The gels can behydrogels, aerogels, or xerogels. A hydrogel is also known as an aquagelwhich is formed in water and as a result its pores are filled withwater. A xerogel is a hydrogel with the water removed. An aerogel is atype of xerogel from which the liquid has been removed in such a way asto minimize collapse or change in the gel's structure as the water isremoved. Silica gels commercially available as Syloid® grade gels, e.g.,grades 74, 221, 234, 244, W300, W500, and Genesis™ silica gels aresuitable parent inorganic oxides.

Gels are well known in the art. See Iler's “The Chemistry of Silica”, p.462 (1979). Gel, e.g. silica gel, particles are distinguishable fromcolloidal silica or precipitated silica particles. For example,colloidal silica is prepared as a slurry of dense, non-porous silicaparticles. Colloidal silica particles typically are smaller than 200 nm(0.2 micron). As mentioned earlier, these particles do not have internalporosity. On the other hand, typical precipitated particles have someinternal porosity. In some cases, the internal porosity in thoseparticles, however, largely collapses under capillary pressure createdby receding menisci of water as the water evaporates during drying. Theconditions for making colloidal silica and precipitated silica are wellknown.

Gels, on the other hand, are prepared under conditions which promotecoalescence of primary particles (typically having median particlessizes of 1 to 10 nm, as measured under by transmission electronmicroscopy, i.e., TEM) to form a relatively rigid three dimensionalnetwork. The coalescence of gel is exhibited on a macroscale when adispersion of inorganic oxide, e.g., silica, hardens to a “gel” or“gelled” mass having structural integrity.

Methods of preparing inorganic oxide gels are well known in the art. Forexample, a silica gel is prepared by mixing an aqueous solution of analkali metal silicate (e.g., sodium silicate) with a strong acid such asnitric or sulfuric acid, the mixing being done under suitable conditionsof agitation to form a clear silica sol which sets into a hydrogel,i.e., macrogel, in less than about one-half hour. The resulting gel isthen washed. The concentration of inorganic oxide, i.e., SiO₂, formed inthe hydrogel is usually in the range of about 10 and about 50,preferably between about 20 and about 35, and most preferably betweenabout 30 and about 35 weight percent, with the pH of that gel being fromabout 1 to about 9, preferably 1 to about 4. A wide range of mixingtemperatures can be employed, this range being typically from about 20to about 50° C.

The newly formed hydrogels are washed simply by immersion in acontinuously moving stream of water which leaches out the undesirablesalts, leaving about 99.5 weight percent or more pure inorganic oxidebehind.

The pH, temperature, and duration of the wash water will influence thephysical properties of the silica, such as surface area (SA) and porevolume (PV). Silica gel washed at 65-90° C. at pH's of 8-9 for 15-36hours will usually have SA's of 250-400 and form aerogels with PV's of1.4 to 1.7 cc/gm. Silica gel washed at pH's of 3-5 at 50-65° C. for15-25 hours will have SA's of 700-850 and form aerogels with PV's of0.6-1.3. These measurements are generated by N₂ porosity analysis.

Methods for preparing inorganic oxide gels such as alumina and mixedinorganic oxide gels such as silica/alumina cogels are also well knownin the art. Methods for preparing such gels are disclosed in U.S. Pat.No. 4,226,743, the contents of which are incorporated by reference.

In general, alumina gels are prepared by mixing alkali metal aluminatesand aluminum sulfate. Cogels are prepared by cogelling two metal oxidesso that the gels are composited together. For example, silica aluminacogels can be prepared by gelling an alkali metal silicate with an acidor acid salt, and then adding alkali metal aluminate, aging the mixtureand subsequently adding aluminum sulfate. The gel is then washed usingconventional techniques.

Another embodiment comprises particles derived from dispersions ofcertain precipitated inorganic oxides. For example, milling certainprecipitated silicas results in dispersions having the porosityproperties described later below. Viscosity of certain precipitates as afunction of mass fraction is illustrated in FIG. 1.

Reinforced precipitated silica such as that described in U.S. Pat. No.4,157,920 can also be used to prepare the particles of this invention.The contents of that patent are incorporated herein by reference. Forexample, reinforced precipitated silicas can be prepared by firstacidulating an alkali inorganic silicate to create an initialprecipitate. The resulting precipitate is then reinforced or “postconditioned” by additional silicate and acid. The precipitate resultingfrom the second addition of silicate and acid comprises 10 to 70% byweight of the precipitate initially prepared. It is believed that thereinforced structure of this precipitate is more rigid than conventionalprecipitates as a result of the second precipitation. It is believedthat even after milling, centrifuging and subsequent drying, thereinforced silicate substantially maintains its network rigidity andporosity. This is in contrast to other reported precipitated silicassuch as those disclosed in U.S. Pat. No. 5,030,286.

Once an inorganic oxide is selected for the porous particle, it isdispersed in a liquid phase to form a parent dispersion. The medium forthe liquid phase can be aqueous or organic. The liquid phase can beresidual water in inorganic oxide gels which have been drained, but notyet dried, and to which additional water is added to reslurry the gel.In another embodiment, dried inorganic oxides, e.g., xerogels, aredispersed in liquid medium. In general, the parent dispersion should bein a state that can be wet milled. In most embodiments, the parentdispersion has a median particle size approximately in the range of 10to 40 microns. However, the size of the parent particles only needs tobe sufficient such that the mill being used can produce a dispersionhaving the desired median particle size at about or below 3 microns. Inembodiments prepared from a drained inorganic oxide gel, the drained gelmay first be broken up into gel chunks and premilled to produce adispersion of particles in the range of 10 to 40 microns.

(2) Milling

The parent dispersion is then milled. The milling is conducted “wet”,i.e., in liquid media. The general milling conditions can vary dependingon the feed material, residence time, impeller speeds, and milling mediaparticle size. Suitable conditions and residence times are described inthe Examples. These conditions can be varied to obtain the desired sizewithin the range of 0.05 to about 3 microns. The techniques forselecting and modifying these conditions to obtain the desireddispersions are known to those skilled in the art.

The milling equipment used to mill the parent inorganic oxide particlesshould be of the type capable of severely milling and reducing materialsto particles having sizes about three microns or smaller, particularlybelow one micron, e.g., through mechanical action. Such mills arecommercially available, with hammer and sand mills being particularlysuitable for this purpose. Hammer mills impart the necessary mechanicalaction through high speed metal blades, and sand mills impart the actionthrough rapidly churning media such as zirconia or sand beads. Impactmills can also be used. Both impact mills and hammer mills reduceparticle size by impact of the inorganic oxide with metal blades. Adispersion comprising particles of three microns or smaller is thenrecovered as the final product. This dispersion can then be added to thebinder and any additives employed.

The milled dispersion may also be further processed. For example,further processing is desirable if there is need to insure thatessentially all of the distribution of particles is below 2 microns, andespecially when dispersions in the size range of 1 micron or less isdesired, e.g., for glossy paper finishes. In such a case, the milleddispersion is processed to separate the dispersion into a supernatantphase, which comprises the particles to be used, and a settled phasewhich comprises larger particles. The separation can be created bycentrifuging the milled inorganic oxide particles. The supernatant phaseis then removed from the settled phase, e.g., by decanting.

Depending on the product particle size targets, the settled phase alsocan be regarded as the particles to be added to the formulation. Forexample, if larger particle sizes within the range of 0.05 to 3 micronsare used for the formulation, the settled phase can be removed andredispersed as the particles which are added to the formulation.

Conventional centrifuges can be used for this phase separation. Acommercially available centrifuge suitable for this invention isidentified in the Examples below. In some instances, it may bepreferable to centrifuge the supernatant two, three or more times tofurther remove large particles remaining after the initial centrifuge.It is also contemplated that the larger particles of a milled dispersioncan separate over time under normal gravity conditions, and thesupernatant can be removed by decanting.

The dispersion of particles also can be modified after milling to insurea stable dispersion. This can be accomplished through pH adjustment,e.g., adding alkaline material, or by the addition of conventionaldispersants.

(3) Properties of Inorganic Oxide Particles

As indicated earlier, the median particle size, i.e., particle diameter,of the porous inorganic particles of this invention is in the range of0.05 to about 3 microns. The size is primarily dictated by theformulation and can be in ranges of, e.g., between 0.06 to 2.9, 0.07 to2.8, and so on. For example, if the particles are to be used for makinga high gloss ink receptive coating, the median particle size willgenerally be less than one micron, and for some typical applications,the dispersion has a median particle size below 0.5 micron, andpreferably in the range of 0.1 and 0.3 micron. The median particle sizeis measured using conventional light scattering instrumentation andmethods. The sizes reported in the Examples were determined by a LA900laser scattering particle size analyzer from Horiba Instruments, Inc.

In general, the properties of the dispersion can be adjusted dependingon the type of coating to be produced from the formulation and the typeof binder to which the particles are to be added.

In general, the dispersion's viscosity should be such that thedispersion can be added to the other components of the formulation. Theviscosity of the dispersion is highly dependent upon the dispersion'ssolids content and the porosity of the particles. The solids content ofthe dispersion is generally in the range of 1-30% by weight, and allranges in between, although in certain applications, the amount can behigher or lower. A solids content in the range of 10 to 20% by weight issuitable for a number of applications. Viscosity enhancers and agentscan also be used to obtain the appropriate viscosity. The viscosity canrange from 1 to over 10,000 centiposes (cp) as measured by a Brookfieldviscometer, e.g., operated at a shear rate of 73.4 sec⁻¹.

Dispersions of particles prepared from silica gel generally haveviscosities similar to the viscosities of the parent silica dispersion.For example, when parent silica gel is milled at a prescribed pH in therange of 8-10, e.g., 9.5, the viscosity of the milled silica remainsrelatively unchanged. This is distinguishable from viscosities of milledprecipitated silicas. The viscosities of milled precipitated silica areless than the viscosity of the parent material.

The pH of the dispersion depends upon the inorganic oxide and additivesused to stabilize the dispersion, and can be adjusted to be compatiblewith the other components in the formulation. The pH can be in the rangeof 2 to 11, and all ranges in between. For example, dispersions ofalumina generally have a pH in the range of 2 to 6. Silica dispersionsare generally neutral to moderately alkaline, e.g., 7 to 11. The pH canalso be modified using conventional pH modifiers.

With respect to a dispersion of particles comprising silica gel, thedispersion is relatively free of impurities when compared to dispersionscomprising, for example, precipitated inorganic oxide particles. Parentsilica gels are typically washed to remove substantially all impurities.The alkali salt content of gels are typically as low as 100 ppm byweight and generally no more than 0.1% based on the weight of gel. Thelow impurity levels of silica gels are especially advantageous whenalkali salt would deleteriously affect the performance of the coating orthe performance of the other components in the formulation.

The pore volume of the particles can be measured on a dry basis bynitrogen porosimetry after the dispersion is dried. In general, at leastabout 0.5 cc/g of the particles' pore volume is from pores having a poresize of 600 Å or less. There are embodiments comprising silica gel inwhich at least 0.7 cc/g and at least 0.9 cc/g of pore volume is frompores having sizes less than 600 Å. In those embodiments, up to about100% of the pores have diameters less than 600 Å, and at least about 80%of the pores in silica gels have diameters of 300 Å or less. The totalpore volume of the particles as measured on a dried basis is in therange of about 0.5 to about 2.0 cc/g, with embodiments comprising silicagel having total pore volume measurements in the range of about 0.5 toabout 1.5, and for certain silica gel embodiments in the range of 0.7 toabout 1.2 cc/g. Measuring the pore size distribution and pore volume ona dry basis requires adjusting the pH of the dispersion of particles toabout 6, slowly drying the dispersion at 105° C. for sixteen hours,activating the dried dispersion at 350° C. under vacuum for two hours,and then using standard BJH nitrogen porosimetry.

The porosity of the particles can also be defined by the viscosity ofthe dispersion system in which the particles are added. Compared to lessporous particles (at the same mass loading in a solvent), porousparticles occupy a greater volume fraction of the solvent-particlesystem and, as such, they to a greater extent disrupt and offer greaterresistance to shear flow of the fluid. FIG. 1 shows that as loadings ofparticles increases, viscosity (η)increases in such a manner that alinear relationship is obtained when $\frac{\eta_{0}}{\eta}$

is plotted against a certain range of particle loadings. η₀ is theviscosity of the dispersion's solvent, i.e., water. As shown in FIG. 1,slope for the curve shown increases as the porosity of particlesincreases. A “viscosity derived pore volume” for the particles thus canbe calculated from the slope of these curves. These values reflect porevolumes for the particles.

For example, the effect of loading small particles on the viscosity of adispersion of those particles in a Newtonian fluid is described by I. M.Krieger in Adv-Coll. Interface Sci., 1972, 3, 111. The formula definesthe reciprocal of $\frac{\eta_{0}}{\eta}$

with the following formula (1). $\begin{matrix}{\frac{\eta}{\eta_{0}} = \left\lbrack {1 - \frac{\Phi}{b}} \right\rbrack^{- {ab}}} & (1)\end{matrix}$

wherein

η is the dispersion's viscosity

η₀ is the viscosity of the fluid in which the particles are dispersed

Φ is the volume fraction of the suspension occupied by the particles

a is the “intrinsic viscosity” (equal to 2.5 for spherical, or very lowaspect ratio uncharged particles)

b is the volume fraction at which the viscosity becomes infinite.

A relationship (2) also exists between Φ and the mass loading (x) ofparticles in the suspension expressed as a mass fraction solids, and theparticles skeletal density (ρs) and its apparent pore volume (PVa),referred to herein as the “viscosity derived pore volume”.$\begin{matrix}{\Phi = \frac{\left( {\frac{1}{\rho \quad s} + {PVa}} \right)\rho \quad {fx}}{1 - {x\left( {1 - \frac{\rho \quad f}{\rho \quad s}} \right)}}} & (2)\end{matrix}$

where ρf is the density of the fluid phase.

Coupling of equations (1) and (2) yields a relationship relating$\frac{\eta_{0}}{\eta}$

to the mass loading of particles. For relatively small values of x thisrelationship can be illustrated by the following linear expression whichis independent of the parameter b. $\begin{matrix}{\frac{\eta_{0}}{\eta} = {1 - {{a\left( {\rho \quad f} \right)}\left( {\frac{1}{\rho \quad s} + {PVa}} \right)x}}} & (3)\end{matrix}$

This linear relationship generally holds for values of$\frac{\eta_{0}}{\eta}$

from 0.5 to 1.0. Viscosity data for a system of well dispersed particlescan then be plotted in the form of $\frac{\eta_{0}}{\eta}(x)$

and linear regression applied to the $\frac{\eta_{0}}{\eta}$

data of 0.5 to 1.0 to determine the slope. From equation (3), it isapparent that this slope can be related to the PVa of the particles bythe following equations. $\begin{matrix}{{slope} = {{- {a\left( {\rho \quad f} \right)}}\left( {\frac{1}{\rho \quad s} + {PVa}} \right)}} & (4) \\{{PVa} = {- \left( {\frac{slope}{a\left( {\rho \quad f} \right)} + \frac{1}{\rho \quad {fs}}} \right)}} & (5)\end{matrix}$

Knowing the skeletal density of amorphous silica (2.1 g/cc), the densityof the fluid phase (water=1.0 g/cc) and knowing that the intrinsicviscosity, a, is equal to approximately 2.5, PVa for the invention iscalculated. This curve is illustrated in FIG. 1 for several embodimentsof the invention, as well as a relatively non-porous colloid.

The viscosity derived pore volume values for dispersions, especiallydispersions of silica particles, are, in general, determined accordingto the following methodology.

(1) A dispersion of selected inorganic oxide is milled at one liter perminute and centrifuged for thirty minutes at 600 G or at 2,000 G.

(2) The pH of the dispersion is then adjusted so that a dispersion isobtained and maintained. Typically this is obtained by adjusting the pHof the dispersion away from the isoelectric point of the particles, butnot into pH regimes that would cause excessive dissolution of theparticles (e.g., for silica adjust the pH to between 9.7 and 10.3 byadding NaOH). In general, this pH range of optimum dispersion can bedetermined by titration of a 5wt. % solids dispersion through the entireregion of acceptably low particle solubility and determining the pHrange associated with minimum dispersion viscosity. The milleddispersion from (1) is then adjusted to a pH in that range.

(3) The viscosity (η) of the dispersion is measured and the viscosity ofthe dispersion's medium (η₀), e.g., water, is determined. Theseviscosities are measured using a Brookfield viscometer at 74 sec⁻¹ at25.0±0.1° C.

(4) The ratio of $\frac{\eta}{\eta_{0}}$

 is then determined to obtain $\frac{\eta_{0}}{\eta}(x)$

 values uniformly dispersed through the range of $\frac{\eta_{0}}{\eta}$

 values between 0.5 and 1.0. This is accomplished by first estimatingthe slope of $\frac{\eta_{0}}{\eta}(x)$

 using a reference sample and then using that estimated slope todetermine the concentration of dispersions to be prepared to give thedesired range of $\frac{\eta_{0}}{\eta}$

 determinations. If $\frac{\eta_{0}}{\eta}$

 of the dispersion from (2) is greater than 0.5 and less than 0.9 it canbe used as the reference sample to calculate the estimated slope, ESL,for the $\frac{\eta_{0}}{\eta}(x)$

 plot. If $\frac{\eta_{0}}{\eta}$

 is less than 0.5, the slurry sample must be diluted with solvent(typically DI water) then reevaluated for $\frac{\eta_{0}}{\eta}.$

 If $\frac{\eta_{0}}{\eta}$

 is greater than 0.9, a more concentrated dispersion sample must beobtained.

Once a reference sample with $\frac{\eta_{0}}{\eta}$

between 0.5 and 0.9 is obtained, the mass loading x is determined usingconventional techniques and ESL is calculated from the followingequation.${ESL} = \frac{{\left( \frac{\eta_{0}}{\eta} \right)\quad {ref}} - 1}{xref}$

(5) Concentrations (x values) for a series of samples for the PVadetermination are then calculated using the following formulae.

${target} = \frac{\eta_{0}}{\eta}$

.9 $x = \frac{{.9} - 1}{ESL}$

.8 $x = \frac{{.8} - 1}{ESL}$

.7 $x = \frac{{.7} - 1}{ESL}$

.6 $x = \frac{{.6} - 1}{ESL}$

.5 $x = \frac{{.5} - 1}{ESL}$

(6) Dispersions with these mass loadings are then prepared within theappropriate pH range determined in (2).

(7) The viscosity of each of these samples is determined by Brookfieldviscometer at a shear rate of 73.4 sec.⁻¹ after equilibration at 25.0±1°C./ These data are then plotted.

(8) Regression analysis is applied to obtain the slope of the datagenerated and the slope, ρs, and ρf are input into the formula$\text{slope} = {{- 2.5}\left( {\frac{1}{\rho \quad s} + {PVa}} \right)\rho \quad f}$

 to calculate (PVa).

Silica dispersions of particles used in this invention show curveshaving an absolute slope of about 2.40 or greater, and generally in therange of 2.4 to 10.0. This data generally translates into dispersionshaving viscosity derived pore volumes (PVa's) of at least about 0.5cc/g. Preferred embodiments of the formulation are prepared fromparticles which show a slope in the range of 3.50-5.0 and have a PVa ofabout 1.0 to about 1.5 cc/g.

The stability of the porosity in the particles of this invention isevidenced by calculating the loss in pore volume after a dispersion ofthe particles is dried. Comparing the particles' PVa and the pore volumemeasured after the dispersion is dried shows that at least 40% of thePVa is maintained for particles of this invention. Certain embodimentsshow that at least about 60% of the PVa pore volume is maintained. SeeFIG. 3 and Example VII. Moreover, embodiments maintaining only 40% ofPVa have a dried pore volume of about 0.5 cc/g or greater.

The inorganic oxide particles can also be surface modified separately toenhance their performance in the formulation, and in particular enhancetheir performance in an ink, receptive coating. These modifications arediscussed later below.

The binder used to prepare the formulation comprises a polymer capableof binding pigment particles. Film forming polymers in general aresuitable. Particularly suitable polymers are those conventionally usedto make formulations for ink receptive coatings and include any one ormore combinations of polyvinyl alcohol derivatives such as completelysaponified polyvinyl alcohol, partially saponified polyvinyl alcohol,silanol group modified vinyl alcohol copolymer; cellulose derivativessuch as carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropylmethyl cellulose, water soluble polymers such as polyvinyl pyrolidone,starch oxide, modified starch, gelatin, casein, or acrylic acid typepolymers. Further, binders such as vinyl acetate and ethylene-vinylacetate emulsions, styrene butadiene latexes, or acrylic type emulsionscan also be used depending on the application.

When the formulation is used as an ink receptive coating, polyvinylalcohol type polymers such as completely saponified polyvinyl alcohol,partially saponified polyvinyl alcohol, or silanol group modified vinylalcohol copolymer are preferable with respect to ink, absorbability andthe strength of the coating.

The formulation optionally comprises additives, depending on how theformulation is being used. In embodiments which are being used toprepare ink receptive coatings, dye mordant additives can be included tofix dyes as they are applied to the coatings. For that purpose, theremay be incorporated in the coating formulation at least one componentselected from the group consisting of amine-containing polymers such aspolyethylene imine or polyaryl amines; ionic water soluble polymers suchas the salts of amine polymers, including poly(diamines) oramine-containing acrylate copolymers or poly(diallyl dimethyl ammoniumchlorides); and water soluble metal salts thereof.

Optional additives also include colorants, thickeners, release agents,flow modifiers, as well as conventional pigments such as clays, fumedsilicas, precipitated silica and the like. Further, a fluorescentbrightening agent, a surfactant, a fungicide, crosslinkers or adispersant can be included in the formulation as required.

The amount of pigment to binder in the formulation varies depending onthe formulations used. For applications such as ink receptive coatingformulations, the weight ratio of pigment to binder in the formulationis 1:100 to 100:1. The ratio depends on the application. If it isdesired to prepare an ink receptive coating formulation in which PVC isless than CPVC, the pigment/binder ratio is typically about 1:1 to about1:50. If a PVC greater than CPVC is desired, the pigment/binder ratio istypically about 1:1 to about 4:1.

Additives typically comprise a smaller percentage of the totalcomposition and generally are added in 1 to 30% by weight of the totalformulation.

The ingredients to the formulation are combined using conventionaltechniques and mixers. The pigment is preferably added as a dispersion,usually comprising about 1-30%, by weight solids, and added in amountsto obtain the desired pigment to binder ratio. The order of additiondepends on the compatibility of the components. If necessary, certainingredients can be precombined with another ingredient before all of theingredients are finally combined together.

The formulation is especially adaptable for preparing ink receptivecoatings and can be applied to substrates used for that purpose. Suchsubstrates include pulp-based substrates including chemical pulps suchas hardwood bleached kraft pulp, softwood bleached kraft pulp, highyield pulps such as groundwood pulp or thermo-mechanical pulp, recycledpulps and non-wood pulps such as cotton pulp can be used. It is possibleto mix synthetic fiber, glass fiber or the like in the pulp depending onthe application.

The substrate can also be films of vinyl, polyester, polystyrene,polyvinyl chloride, polymethyl methacrylate, cellulose acetate,polyethylene and polycarbonate. The substrates for ink receptive mediumgenerally are 10-300 microns in thickness.

The formulation is applied to the substrate using conventionaltechniques, including using conventional coaters such as a blade coater,air knife coater, roll coater, brush coater, curtain coater, bar coater,gravure coater, and spray gun. The recording sheet carrying a freshlyapplied ink receptive layer can be used as such in the ink jetrecording, or after having been improved in surface smoothness bypassing through the roll nip of a super calender, gloss calender, or thelike under application of heat and pressure. The formulation is appliedat an amount in the range of 2 to 50 g/m² and preferably 5 to 30 g/m².The amount of formulation applied should be at least enough to provideacceptable printability and image quality. For ink receptive layers, theformulation generally is applied to a thickness of 1-100 microns.

As mentioned above, the formulation is especially suitable for preparingink receptive coatings, or ink jet paper for which relatively high glossfinishes are desired. In contrast to non-porous prior art colloidalsilica, the volume of the same mass of porous silica provides anadditional source volume, i.e., internal pore volume. This is reflectedby V_(porous silica)=m*(1/ρ+PV) where PV is the pore volume of the driedsilica, or void space associated with the network of pores internal tothe silica particles, and where ρ is the silica skeletal density, i.e.,2.1 g/cc. PV is measured by nitrogen BJH porosimetry. The volume of agiven mass, m, of non-porous colloidal silica is given byV_(colloid)=m/ρ.

This internal pore volume is especially advantageous for ink receptivecoatings. In the ink-receptive coating application of this invention,the porous inorganic oxide pigment is formulated with binders that, forthe most part, do not absorb into internal void spaces, thereby makingpore volume available for absorption of ink-fluids. By contrast,colloidal silica does not contain this internal void space, and coatingscomprising colloid become saturated with low levels of ink-liquids. Withall other factors being equal, a coating formulated with porousinorganic oxides has an additional capacity for ink fluid absorptionrelative to coatings formulated with prior art colloidal particles, theadditional capacity being directly related to pore volume. In a similarfashion, ink-drytimes for coatings formulated with porous pigments areusually shorter than those coatings formulated with colloidal silica.

Unlike formulations comprising typical pigments, the inorganic oxidepigment of the formulation also has porosity that is stable. Theexamples below illustrate that the porosity of the particles of thisinvention is maintained after drying. Silica gel particles show furtherstability as evidenced by the maintenance of porosity when the particlesare formed by milling. See Examples VI and VII. Therefore, fromformulation to formulation, the invention consistently contributesproperties such as fast ink dry times, minimal dot gain (spreading),good image resolution, high ink loads and exceptional color gamut.

Even further, the use of this formulation results in high qualityink-receptive coatings that exhibit higher gloss than coatings preparedusing larger size prior art pigments that are formulated above the CPVC.Those prior art pigments are used primarily for ink receptivity andtypically do not exhibit very high gloss because the surface of thecoating has a degree of roughness associated with it. For coatingsformulated using prior pigments above the CPVC, the surface roughness isrelated to the particle size of the “continuous” or close-packed pigmentphase. In comparison to coatings formulated with relatively largeparticles of porous silica, glossy coatings can still be maintained withthe small, porous inorganic oxide particles of this invention becausethey do not create the same degree of surface roughness.

This invention also provides acceptable ink receptivity and relativelyglossy coatings at low PVC, specifically for coatings having a PVC lessthan CPVC. The advantage of the fine porous particles of this inventioncompared to colloidal silicas in this circumstance is that they increasethe capacity of the coating to absorb liquid. The increased capacity toabsorb ink liquids is directly related to the intrinsic porosityinternal to each porous particle.

As mentioned earlier, the surface of the fine particles can be modifiedto enhance their performance, especially in ink receptive coatings. Thefine inorganic particles of the dispersions can be modified to createparticles exhibiting positive surface charge (zeta potential). Thesurface charge should have a zeta potential of at least +20 mV, andpreferably at least +40 mV.

Ink receptive coating formulations typically have a pH in the range of 2to 8. Dispersions comprising particles having a positive surface chargeare more stable towards irreversible agglomeration in that pH range thanare the unmodified dispersions, especially if silica is used for toprepare particles and the silica particles exhibit a negative zetapotential. Ink-receptive coatings prepared from these formulations alsoshow better image-forming characteristics. To create the positivesurface charge on the particles, the particles can be modified byadditives having a cationic moiety and can be modified, for example,with alumina, organic cation-containing silanes, e.g., amine-containingsilanes, and ionic polymers, e.g., quaternary ammonium compounds such asdiallyl dimethyl ammonium chloride polymer. The particles can bemodified by introducing the modifying additive when the parent inorganicoxide is dispersed, e.g., co-milling alumina or cationic polymer with aparent silica dispersion. The particles can also be modified by reactingthe particles with additive after the particles are made, e.g.,conducting a silanization reaction with amino silanes. The examplesbelow show that irreversible agglomeration of the particles in thedispersion is reduced or eliminated when modifying the particles asdescribed above.

The following examples of the invention are illustrative and are notintended to limit in any way the invention as recited in the appendedclaims.

ILLUSTRATIVE EXAMPLES Preparation of Inorganic Oxide Particles Example ISilica Gel Particles Derived from Hydrogel Parent

Well drained hydrogel¹ was presized by a Prater mill to a medianparticle size of approximately 30 μ. The powder was then slurried indeionized water (DI) yielding a slurry of about 20% by weight solids andpH of about 8. This slurry was fed to a five (5) liter Drais media mill(model PM5RLH, 1.5 mm, glass media) at a rate of one liter per minuteresulting in a viscous slurry.

Separation of the coarse and fine (submicron) fractions of the milledsilica gel product was accomplished by a two step centrifugation process(90 min. at 1400 G's, decant, then 40 min. at 2,000 G's). The finalsubmicron particle suspension was obtained by simply decanting. Thesolids content of the supernatant dispersion was 13 wt. % and yield wasdetermined to be 41% (on a dry SiO₂ basis).

¹Hydrogel prepared at about 1.5 pH to produce 19% by weight silica andwashed with dilute NH₄OH.

Particle Size Distribution (Horiba 900)² 10%< .13μ 50%< .22μ 90%< .38μ99.9%<   .77μ ²Determination of particle size distribution requiredseparation of coarse and fine fractions by centrifugation, particle sizemeasurement of each fraction by Horiba Instruments 900 brand particlesize analyzer, and then constructing the composited distribution byweight summation.

Example II Silica Gel Particles Derived from Hydrous Gel Parent

Another submicron silica gel product was made using the same process asdescribed in Example I except that the parent gel was presized in an airclassification mill yielding a median silica gel particle ofapproximately 15 μThe gel is partially dried during this process withits moisture content (measured as total volatiles) dropping from about67% to 55% by weight thus forming a hydrous gel material.

After media milling and centrifugation as described in Example I (exceptat 27% solids vs. 20% in Ex. I), a supernatant comprising a dispersionof 12 wt. % solids at a yield of 10% was obtained. The supernatant hadthe following particle size distribution:

Particle Size Distribution (Horiba 900) 10%< .13μ 50%< .18μ 90%< .30μ  99.9<   .55μ

Example III Silica Gel Particles Derived from Aerogel Parent

Wet-milled Genesis™ gel was slurried to approximately 20% solids byweight in deionized water and the pH was adjusted to about 8. The slurrywas then wet milled using a Netzsch LMZ-11 mill (with 0.6-0.8 mm SEPRmedia) at 3.8 liters per minute. The milled slurry was then diluted to14.9% solids with DI water using a Myers mixer.

Separation of the coarse and fine fractions of the milled gel wasaccomplished by a two step centrifugation process, i.e., 90 minutes at1,050 G's, decant, then another spin at the same conditions.

The total solids was 8.8% and the particle size distribution was:

Particle Size Distribution (Horiba 900) 10%< .086μ 50%< .121μ 99%< .181μ99.9%<   .322μ

Example IV Silica Gel Particles Derived from Xerogel Parent

Syloid® 74×6500 silica xerogel was slurried in D.I. water to produce a24% by weight solids dispersion, and NH₄OH was added to adjust the pH toabout 8.

This slurry was then wet-milled using a Netzsch LMZ-05 mill (with 0.4 to0.6 mm SEPR media) and a recirculation rate of 0.75 L/min. The totalbatch was passed through the mill six times. The pH after milling was8.20. The final particle size of the milled slurry was:

Particle Size Distribution (Horiba 900) 10%< 0.72μ 50%< 1.30μ 99.9%<  4.59μ

Example V Viscosity Derived Pore Volumes (PVa) and Dried Dispersion PoreVolumes of Assorted Inorganic Oxide Particles

Sample1—Hydrous Gel

A hydrous gel having fifty-five (55) weight % total volatiles wasslurried to 19% by weight solids. The pH was adjusted to 9.6 with NaOH.The dispersion was milled in a four liter Drais mill (1.5 mm glassbeads) at a rate of 1 liter (L)/minute using six passes.

The resulting slurry was then centrifuged for thirty minutes at 600 G,2000 G, or 27,000 G. Viscosity derived pore volumes (PVa), dried porevolumes (N₂ BJH porosimetry), as well as particle size distribution andBET surface areas (nitrogen porosimetry) were measured for the parentdispersion, milled dispersion and each of the centrifuged dispersions.The results are reported in Table 1 below.

Sample 2—Precipitated Silica

A dispersion of 11.4% solids was prepared using FK310 precipitatedsilica from Degussa. The pH of the dispersion was adjusted to 9.3 andthen milled, centrifuged, measured and tested in the same manner asSample 1. The results are reported in Table 1.

Sample 3—Silica Gel

A dispersion of 21.4% solids was prepared using Syloid® 63 silica gelfrom Grace Davison of W. R. Grace & Co.-Conn. The pH of the dispersionwas adjusted to 9.8.

The dispersion was then milled (except for 8 passes instead of 6),centrifuged, measured and tested in the same manner as Sample 1. Theresults are reported in Table 1.

Sample 4—Precipitated Silica

A dispersion of 8.4% solids was prepared using Zeothix™ 177 precipitatedsilica from Huber. The dispersion was then milled (using Netzsch mill),centrifuged (except only at 2000 G's for thirty minutes), measured andtested in the same manner as Sample 1. The results are reported in Table1.

Sample 5—Aerogel

A dispersion of 18.2% solids was prepared from Genesis™ gel from GraceDavison. The pH of the dispersion was adjusted to 9.8. The dispersionwas milled in a Reitz mill (0.016 screen) for three passes and thenmilled eight more times in a Drais mill. Both mills were fed withinorganic oxide at one liter/minute. The milled dispersion was thencentrifuged, measured and tested in the same manner as described inSample 1. The results are reported in Table 1.

Sample 6—Colloidal Silica

A sample of Nalco 1140 colloidal silica available from Nalco wasmeasured and tested in the same manner as described for Sample 1. Themedian particle size of 0.015 micron is taken from literature availablein the art. The results are reported in Table 1.

TABLE 1 Viscosity N2 Porosimetry @ (1) 10% Viscosity Derived (2) onNeutralized/Dried % (Weight) Particle Size (Honba 900), μ Solids, 25° C.Pore Volume (PVa) PV @ PV @ Solids pH 10% 50% 90% 99.90% (cp) (cc/g) SA967 P/Po 995 P/Po Sample 1 Hydrous Gel Parent Slurry 18.5 9.6 4.4 8.815.8 32.1 1.82 1.31 265 1.163 1.179 Drais Milled 19 9.6 36 60 1.6 4.21.82 1.33 215 0.932 0.936 Milled & Centr 30 mins @  600 g 17.2 9.4 29 4484 2.7 1.83 1.33 230 0.962 0.962  2000 g 15.8 9.2 23 38 71 2.3 1.95 1.47236 1.051 1.054 27000 g 10.1 9.3 .08 11 15 24 1.93 1.45 252 0.859 0.868Sample 2 FK-310 Precipitate Parent Slurry 11.4 9.3 3.4 6.6 11.3 21.62.41 1.86 330 1.141 1.423 Drais Milled 11.9 9.5 37 68 1.9 5.4 1.81 1.31233 0.878 1.113 Milled & Centr 30 mins @  600 g 7.5 9.8 20 33 55 1.61.88 1.40 288 1.122 1.254  2000 g 4.8 9.5 12 20 36 78 1.80 1.30 2681.070 1.072 27000 g 0.4 9.5 — — — — Sample 3 Syloid ® 63 Silica ParentSlurry 21.4 9.8 2.5 7.5 16.3 35.7 1.31 48 326 366 368 Milled (Drais)21.5 9.8 26 66 1.7 4.5 1.41 68 212 298 386 Centr 600 g 13.6 9.9 24 37 551.2 1.43 72 212 423 572 2000 6 9.9 11 16 24 45 185 666 716 27000 0.5 — —— — Sample 4 Zeothix 177 Precipitate Parent Slurry 8.4 9.9 1.5 3.7 7.115.6 10.5 3.14 109 408 598 Milled (Netzsch) 15.7 9.7 29 59 4.2 1.83 1.33130 696 855 Center (2000 g) 14.6 9.6 0.14 26 50 1.8 1.83 1.33 145 827956 Sample 5 Genesis Gel Parent Slurry 18.2 9.8 6.7 29.4 70.3 143.3 2.251.75 267 1.140 1.155 (Reitz Milled) Drais Milled 18.5 9.8 0.35 0.65 2.432.8 2.25 1.75 246 0.968 0.972 Milled & Centr 30 mins @  600 g 17.5 9.80.28 0.48 1.7 6.1 2.25 1.75 266 1.062 1.073  2000 g 16.6 9.8 0.20 0.421.3 4.0 2.25 1.75 263 0.998 1.003 27000 g 13.2 9.8 0.09 0.14 0.26 0.78 —— 265 0.979 0.985 Sample 6 Colloidal Silica Nalco 1140 40.8 9.9 (015)1.28 40 155 403 405 (1) Interpolated from regression analysis (FIG. 1)(2) Calculated from viscosity data

FIG. 1 reflects the viscosity and mass fraction solids data plotted todetermine the PVa for dispersions described in Samples 1, 2, 3, 4 and 6.This Figure confirms PVa measurements calculated using the methodologydescribed earlier. Viscosity and loadings data for the dispersion ofSample 1 centrifuged at 600 G is reflected by () in FIG. 1. The samedata for the dispersions of Samples 2 and 3 centrifuged at 600 G isreflected by (□), and (◯), respectively. The data for the dispersion ofSample 4 was from the dispersion centrifuged at 2000 G and is reflectedby (Δ) in FIG. 1. The data for Sample 6, as is, is reflected by (▪) inFIG. 1.

The slopes of the curves in FIG. 1 were calculated using regressionanalysis and inserted in formula (3) illustrated earlier along with theadditional data below to determine PVa's.

η, η₀ was determined using a Brookfield LVTD viscometer using a jacketedlow viscosity cell controlled at 25.0 to 0.1° C., at a shear rate of73.4/sec.

α2.5 assumed for spherical particles

ρf 1.0 g/cc for water

ρs skeletal density of inorganic oxide, e.g., 2.1 g/cc, for silica

Example VI Viscosity Derived Pore Volume of Silica Particles

Sample 1

A Brookfield viscometer at 73.4 sec⁻¹, viscosity (cps) was used tomeasure the parent dispersion, the Drais milled dispersion andcentrifuged (600 G) dispersion of Sample 1 (hydrous gel) of Example Vand plotted as (η) in $\frac{\eta_{0}}{\eta}$

versus mass fraction solids wherein η₀ is the viscosity of water. Thedata for the parent (◯), milled dispersion (□), and centrifuged (Δ)dispersions is illustrated in FIG. 2A. The median particle size and PVafor each were 8.8μ and 1.34, 0.60μ and 1.33, and 0.44μ and 1.33,respectively.

Sample 2

Viscosity (cps) was measured (using Brookfield at 73.4 sec⁻¹) for theparent dispersion, milled dispersion and centrifuged (2000 G) dispersionof Sample 4 (Zeothix™) in Example V and plotted as (η) in$\frac{\eta_{0}}{\eta}$

versus mass fraction solids wherein η₀ is the viscosity of water. Thedata for the parent (◯), milled dispersion (□), and centrifuged (Δ)dispersions is illustrated in FIG. 2B. The median particle size and PVafor each were 3.7μ and 3.14, 0.59μ and 1.33, and 0.26μ and 1.33,respectively.

Sample 3

Viscosity (cps) was measured (using a Brookfield viscometer at 73.4sec⁻¹) for the parent dispersion, milled dispersion and centrifugeddispersion (600 G) and plotted as (η) in $\frac{\eta_{0}}{\eta}$

versus mass fraction solids wherein η₀ is the viscosity of water. Thedata for the parent (◯), milled dispersion (□), and centrifuged (Δ)dispersions is illustrated in FIG. 2C. The median particle size and PVafor each were 6.6μ and 1.86, 0.68μ and 1.31, and 0.33μ and 1.40,respectively.

FIG. 2A illustrates that the parent, milled and centrifuged dispersionsof silica gel have about the same viscosity, and accordingly similarPVa's. This indicates that pore volume was not measurably lost when theparent silica gel dispersion was milled. FIGS. 2B and 2C show thatprecipitated silicas of this invention have a reduced viscosity comparedto their parent at comparable loadings after milling. This is believedto be caused by destruction of pore volume.

Example VII Maintenance of Pore Volume Upon Drying

The N₂ BJH pore volume measured for the dispersions made in Example VIwere compared and plotted against the PVa measured for thosedispersions. This comparison is illustrated in FIG. 3. The dispersionswere pH adjusted to 6, dried at 105° C. for about 16 hours, activated at350° C. for two hours and then measured using BJH nitrogen porosimetry.

The dashed (-) line is a line of comparison where the BJH pore volumeequals PVa. This line reflects no loss of porosity upon drying. Theother data reflected in FIG. 3 is identified in the following legend.

 ID (Sample 1)

◯ Degussa (Sample 2)

Δ Huber Zeothix 177 (Sample 4)

□ Syloid 63 (Sample 3)

▪ Nalco 1140 (Sample 6)

B=Parent slurry unmilled

M=Milled slurry not centrifuged

6=Colloidal supernatant after centrifuged at 600 G

20=After centrifuged at 2000 G

The upper point of data is pore volume calculated @.985 P/Po and thelower point is pore volume calculated at 0.967 P/Po.

The data for Syloid 63 silica gel (□) reflects that the inventivedispersions maintain at least 40% of PVa after drying. Other silicadispersions, e.g., ID gel (), maintains at least 60% of PVa. This dataand data showing that at least 0.5 cc/g of porosity is from pores havingsizes below 600 Å indicates that the porosity is internal porosity whichis less subject to the factors that affect prior art dispersions.

Example VIII Glossy Paper Coatings—Preparation of Coatings with ImprovedGloss Compared to Prior Art Silica Gels

Starting Materials:

(a) A dispersion of sub-micron silica particles was produced by aprocess similar to that described in Example I. The total solids of thisslurry was 16.0% by weight. The particle size of this sample was:

Horiba Particle Size D₁₀,μ 0.193 D₅₀,μ 0.339 D₉₀,μ 0.584 D_(99.9),μ1.670

(b) A dispersion of SYLOID® W300 silica gel (Grace Davison), totalsolids of 45% by weight, was used for comparison. This product has anaverage particle size (D₅₀) of about 8μ.

(c) A latex (Vinac XX210, non-ionic polyvinylacetate latex, availablefrom Air Products) was used as the binder.

(d)The substrate was a conventional gloss white film.

Procedure: Coating formulations were prepared at constant solids contentand silica/binder ratio, so that the effect of silica particle size onfilm gloss could be determined. The silicas were mixed into the latex,and this formulation was coated onto the white film using a K ControlCoater and a #6 rod. The wet coatings were dried using a heat gun, andthen were heated in an oven at 80° C. for 5 minutes. Gloss measurementswere made using a Byk-Garner Gloss Meter on the coated sheets at 20°,60° and 85° from normal. High values correspond to high gloss. Resultsare given in table below.

It can be seen that use of the sub-micron silica resulted in coatingswith gloss higher than for those with W300 silica which has a medianparticle size of 8 μ

Sample Coating Silica/binder Gloss Number Silica Solids (by weight) 20°60° 85° 1 submicron silica 37 0.28 1.3 8.1 32.7 2 submicron silica 320.46 1.4 10.3 62.0 3 W300 37 0.28 1.2 4.1 6.0 4 W300 32 0.47 1.2 3.0 4.2

Example IX Improved Ink Drytime Over Non-Porous Colloidal Silica

Starting Materials:

(a) A dispersion of sub-micron silica particles was produced by aprocess by wet-milling W500 silica at 18.6% total solids using theNetzsch LMZ-11 media mill charged with 0.6-0.8 mm media. The pH of thissuspension was 8.6, and the particle size of this sample was:

Horiba Particle Size D₁₀, μ 0.318 D₅₀, μ 0.512 D_(99.9), μ 3.18

(b) The milled slurry was then centrifuged at 1060 G for 30 min. Therecovered supernatant had a solids content of 17.4%, and the particlesize was:

Horiba Particle Size D₁₀, μ 0.254 D₅₀, μ 0.403 D_(99.9), μ 2.334

(c) A sample of Nalco 1140 from Example V (Sample 6) was used as thenon-porous silica.

Procedure:

Coating formulations were prepared at constant solids and constantsilica/binder ratio, so that the effect of silica porosity on inkdry-time could be measured. The formulation used for comparison was 100parts silica, 30 parts poly(vinylalcohol) [Air Products Airvol 823] and15 parts poly(diallyl dimethyl ammonium chloride) dye mordant [CalgonCP261L V]. Silica dispersions having 17.4% solids were prepared, andthen charged to a mixer, and the pH was lowered with the addition of 1.0M HCl to 2.8-3.3. The Airvol 823 was then added, and the silica/PVOHmixture was stirred for 1-2 min. Finally, the CP261LV mordant, afterdilution with water, was added dropwise with vigorous stirring. Thefinal pH was adjusted to between 2.8 and 3.5.

The formulation was coated onto a film substrate (ICI Melinix #454)using a K Control Coater and a #8 rod. The wet coatings were dried usinga heat gun, and then were heated in an oven at 80° C. for 5 min. Visualexamination of the films demonstrated that they were free fromlarge-scale defects.

In order to measure ink-drytime, a Hewlett-Packard 550° C. printer wasused to print a black strip of ink down the length of the coated film.After intervals of ˜1 min., a strip of paper was laid over the printedarea and pressed with a roller of fixed mass. The amount of inktransferred from the film to the paper was then observed visually. Thetime at which there was essentially no ink-transfer is given below foreach of samples (a)-(c):

Sample (a)- milled W500: 2 min. < t < 4 min. Sample (b)- milled,centrifuged W500: 2 min. < t < 4 min. Sample (c)- Nalco (nonporous)silica: 4 min. < t < 6 min.

Thus, the film was dry between 2 and 4 minutes for the porous silicacoatings, but took longer to dry for the nonporous silica coating.

Example X Improved Ink Drytime Over Non-Porous Colloidal Silica

Starting Materials:

The same silicas used in Example IX were used in this Example.

Procedure:

Coating formulations were prepared at constant solids and constantsilica/binder ratio, so that the effect of silica porosity on inkdry-time could be measured. The formulation used for comparison was 69parts silica, 21 parts poly(vinylalcohol) [Air Products Airvol 325] and10 parts poly(ethyleneimine) dye mordant [BASF Lupasol G35]. The silicadispersions of 17.4% solids were prepared for each sample and thencharged to a mixer, and the pH was lowered with the addition of 1.0 MHCl to 2.8-3.3. The Airvol 325 was then added, and the silica/PVOHmixture was stirred for 1-2 min. Finally, the Lupasol G35 mordant, afterdilution with water, was added dropwise with vigorous stirring. Thefinal pH was adjusted to between 2.8 and 3.5.

The formulation was coated onto a film substrate (ICI Melinix #454)using a K Control Coater and a #8 rod. The wet coatings were dried usinga heat gun, and then were heated in an oven at 80° C. for 5 min. Visualexamination of the films demonstrated that they were free fromlarge-scale defects.

Ink drytimes were measured as in Example IX. They were:

Sample (a)- milled W500: 4 min. < t < 5 min. Sample (b)- milled,centrifuged W500: 5 min. < t < 6 min. Sample (c)- Nalco (nonporous)silica: 6 min. < t < 7 min.

Thus, the film was dry between 4 and 6 minutes for the porous silicacoatings, but took longer to dry for the nonporous silica coating.

Example XI Improved Capacity

Formulations comprising milled W500, and milled and centrifuged W500described in Example IX are made at 80 parts pigment and 20 partsbinder, and applied to a vinyl substrate and allowed to dry under theconditions described in Example IX. The coating is removed from thesubstrate and measured for porosity using BJH nitrogen porosimetry. Thepore volume measurements show that such coatings have an ink capacity of10.2 cc per 10 grams of coating. Other coatings can be prepared to haveink capacities in the range of 3 to 50 cc per 10 grams of coating, andall other ranges in between.

A formulation and coating is similarly made with the Nalco colloidalmaterial described in Example IX. The coating is dried, removed from thesubstrate and porosity for that coating is measured. Such coating has anink capacity of 2.2 cc per 10 grams and generally such coatings have acapacity of less than 3 cc per 10 grams.

Example XII Alumina Modification of Finely Divided, Porous Silica Gel

A dispersion of 18% solids was prepared as follows.

434 gms (as is basis) of Syloid 74×6500 were dispersed in 1800 gm DIwater. 140 gms of aluminum chlorohydrol (23% by weight Al₂O₃) were addedwith intensive mixing. 10 cc of 2M NaOH were then added. This mixturewas wet-milled using a Netzsch LMZ-05 media mill with 0.6-0.8 mm SEPRmedia. The dispersion was passed through the mill a total of eight timesat a flow rate of 0.6 kg/min. The product pH was 3.2, and the particlesize was:

Horiba 900 Particle Size D₁₀ 0.44 D₅₀ 1.02 D_(99.9) 7.83

The zeta potential for this sample was determined to be about +22 mVusing Coulter Delsa 440SX electrophoretic mobility analyzer. The samplewas allowed to sit for one month and then redispersed by mixing at 2000RPM for 2 min. using a 60 mm dia. Cowles blade in a 118 mm diametercontainer, and the particle size was again measured to be

Horiba 900 Particle Size D₁₀ 0.41 D₅₀ 1.09 D_(99.9) 17.8

The particles are thus relatively stable towards irreversibleagglomeration at this pH as evidenced by the nearly constant D₅₀ values.

Comparison to Example XII Unmodified Silica Dispersion

By comparison to Example 1, if a silica dispersion is made in the samefashion as above without the alumina source, the charge on the silicaparticle surface will be negative. If the pH is adjusted to be near thepH of the above dispersion (3-4), the low pH dispersion willirreversibly gel within a matter of a few weeks.

Example XIII 3-Aminopropyltriethoxysilane Modification of FinelyDivided, Porous Silica Gel

A dispersion of 18% solids was prepared as follows.

50 g (as is basis) of finely divided, porous Syloid 244 silica geldispersion (20% solids) were acidified (pH=2.8-3.5) using an 1.0 Nhydrochloric acid solution. In a separate container 2.2 ml of 1.0Nhydrochloric acid were diluted with 3 mL D.I. water and to that 0.5 g of3-Aminopropyltriethoxysilane (Dow Corning Z-6011) were added. Thissilane solution was added to the above porous silica gel slurry. Theproduct pH was 2.8-4.5, and the particle size measured immediately afterpreparation was:

Horiba 900 Particle Size Starting Porous Treated Porous SilicaDispersion Silica Dispersions D₁₀ 0.27 0.27 D₅₀ 0.48 0.47 D_(99.9) 5.265.30

The zeta potential for the treated sample was determined to be about +40mV.

Comparison to Example XIII Unmodified Silica Dispersion

By comparison to Example 2, if a silica dispersion is made in the samefashion as above without the addition of 3-aminopropyltriethoxysilane,the charge on the silica particle surface will be negative. The silicaparticles are also prone to agglomeration as shown for the particle sizemeasurements seven days after the preparation of the samples:

Horiba 900 Particle Size Starting Porous Silica Dispersion TreatedPorous Silica Dispersion D₁₀ 7.16 0.28 D₅₀ 13.1 0.48 D_(99.9) 314.8 6.14

The particles are thus relatively stable towards irreversibleagglomeration at this pH as evidenced by the nearly constant D₅₀ values.Four weeks later the untreated sample irreversibly agglomerates to a gellike mass whereas the silane treated example shows particle sizedistribution similar to the fresh prepared samples.

Example XIV Ink Jet Coating Formulation Using the3-Aminopropyltriethoxysilane Modified Porous Silica Dispersion

48.5 g of finely divided, porous silica gel slurry (20.6 wt. % solids)was placed in a container equipped with an overhead stirrer. The slurrywas acidified (2.8-3.5) with 1.0 N hydrochloric acid. In a separatecontainer 2.3 ml of 1.0 N hydrochloric acid were diluted with 6.1 mLD.I. water and to that 0.5 g of 3-aminopropyltriethoxysilane (DowCorning Z-6011) were added. This silane solution was added to the aboveporous silica gel slurry. To that 30.0 g of a polyvinyl alcohol solution(10 wt. % solids) was added. The pH of the mixture was readjusted to2.8-3.5 with 1.0 N hydrochloric acid. A solution containing 3.75 g of261® LV (40 wt. %; Calgon Corp.) diluted with D. I. water (6.0 g) wasadded to the above mixture. The final pH of the coating formulation was2.8-3.5. PET transparent films were coated via depositing a 100μ wetfilm. After drying the resulting coating has a smooth, glossy appearancewith very good printability using dye or pigmented inks.

Comparison to Example XIV Ink Jet Coating Formulation Using UnmodifiedPorous Silica Dispersion

By comparison to Example 3, if an ink jet formulation is made in asimilar fashion as above without the addition of3-aminopropyltriethoxysilane, agglomeration of silica occurs resultingin a gel like formulation that breaks down only under very high sheeryielding a gritty coating.

Example XV Polymer Modification of Finely Divided, Porous Silica Gel

A dispersion of 16% solids was prepared as follows.

435 gms (as is basis) of Syloid 221 silica were dispersed in 1800 gm DIwater. 100 gms diallyl dimethyl ammonium chloride polymer (CalgonCP261LV, poly [dadmac]) were added with intensive mixing. 10 cc of 3.5 MHCl were then added. This mixture was wet-milled using a Netzsch LMZ-05media mill with 0.6-0.8 mm SEPR media. The dispersion was passed throughthe mill for a total of 45 min. at a flow rate of 0.67 L/min., withrecycle of mill product into the mill feed tank. The product pH was 3.2,and the particle size was:

Horiba 900 Particle Size D₁₀ 0.35 D₅₀ 0.61 D_(99.9) 4.17

Three weeks later, the particle size for this sample was remeasured tobe:

Horiba 900 Particle Size D₁₀ 0.33 D₅₀ 0.64 D_(99.9) 4.70

The particles are thus relatively stable towards irreversibleagglomeration and this pH as evidenced by the nearly constant particlesize values.

What is claimed:
 1. A dispersion comprising porous inorganic oxideparticles having (c) a median particle size in the range of 0.05 toabout 3 microns; and (d) porosity such that when an aqueous dispersionof the particles is dried at least about 0.5 cc/g of pore volume asmeasured by BJH nitrogen porosimetry is from pores having a pore size of600 Å or smaller; wherein the inorganic oxide particles have a zetapotential of at least +20 mV and said particles possess a viscosityderived pore volume (PVa) of at least 0.5 cc/g as governed by therelationship: slope=2.5({fraction (1/ρs)}+PVa) ρf  where the slope isabout 2.4 or greater, ρs is the particles skeletal density and ρf is thedensity of dispersion fluid phase.
 2. A dispersion of claim 1, whereinthe inorganic oxide particles have a zeta potential of at least +40 mV.3. A dispersion of claim 1 wherein the inorganic oxide particles aresilica particles which have been modified by a member of the groupconsisting of alumina, cation-containing silane and cationic polyxuer.4. A dispersion of claim 1, wherein the slope is in the range of 2.4 to10.0.
 5. A dispersion of claim 1, wherein the porous inorganic oxideparticles have a viscosity-derived pore volume in the range of about 1.0to about 1.5 cc/g.
 6. A dispersion of claim 1, wherein at least 40% ofsaid viscosity derived pore volume of said dispersion is maintainedafter said dispersion is dried.